There is great concern about climate change that is driven by the use of man made materials. Refrigerants, solvents and blowing agents have been identified as a class of materials that have a high potential for contributing to climate change. This is because materials previously used in these applications have been a class of compounds whose atmospheric life is long enough to increase global warming and/or ozone depletion.
A new class of compounds has been discovered as replacements for the former compounds. This class of compounds is generically known as hydrofluoroolefins (HFOs). HFOs are suitable for use as refrigerants, solvents and blowing agents and have short atmospheric lifetimes. The lifetimes are such that they are minimal contributors to global warming and/or ozone depletion, and these compounds can meet the guidelines being proposed of legislated for such applications.
Trans HFO-1234ze (trans-1,3,3,3-tetrafluoropropene) is one such material. Its global warming potential will meet all currently proposed guidelines. However, some fluoroolefins are known to be toxic or otherwise undesirable. Trans HFO-1234ze has been tested for toxicity and found to be generally non-toxic and very suitable for use as a blowing agent, refrigerant or solvent. Accordingly, the presence of other fluoroolefins, even at low concentrations, may be a cause for concern unless they have been tested for toxicity. During the production of trans HFO-1234ze, the present inventors have observed the presence of at least one other isomer of HFO-1234 (C3H2F4), such as HFO-1234zc, whose toxicity is unknown. In view of the unknown toxicity, the desire to remove this impurity and other impurities (e.g., other HFO compounds) from the product stream was formed. These impurities are collectively referred to herein as “the HFO-1234 impurity.”
There are many separation techniques that are available. Distillation is standard and very efficient means of separation. However, in cases where the volatility of the impurity is similar to that of the product, standard distillation techniques may not work. It may then be necessary to apply more complicated distillation techniques such as the use of multiple columns and even cryogenic techniques; see for example, U.S. Pat. No. 5,261,948. These techniques can be expensive and/or difficult to use. Alternatively, it would then be desirable to apply adsorptive techniques, if a suitable sorbent can be identified.
One of the means of purification of gas streams is to utilize molecular sieves. However, there are many kinds of molecular sieves with varying pore sizes. A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids. Molecules small enough to pass through the pores are adsorbed while larger molecules are not. The principle of absorption to molecular sieve particles is somewhat similar to that of size exclusion chromatography, except that without a changing solution composition, the adsorbed product remains trapped because in the absence of other molecules able to penetrate the pore and fill the space, a vacuum would be created by desorption.
Often molecular sieves consist of aluminosilicate minerals, clays, porous glasses, microporous charcoals, zeolites, active carbons, or synthetic compounds that have open structures through which small molecules can diffuse.
Separations using molecular sieves are partly dependent on the size of the molecules of the gas components. However, different means of estimating molecular sizes yield different results. This makes it impossible to be sure that a particular sieve will produce the required separation. It is also true that many important zeolite based separation processes are not based on the sieving action of the zeolite. They are instead based on the difference between the equilibrium amounts of the gas components adsorbents. See, Gas Separation by Adsorption Processes, by R. T. Yang, Butterworth Series in Chemical Engineering, 1987. Indeed it is possible that the differences in amounts of gas components adsorbed can be such as to increase the concentration of the unwanted impurity.
Another feature that can control the separation process is the kinetics of the materials to be separated. Foley at al., in U.S. Pat. No. 5,261,948, teach that oxygen and nitrogen differ in size by only 0.2 angstrom and the equilibrium loading levels of the two gases are almost identical. Nevertheless, their separation using carbon molecular sieves is efficient. This separation depends on the fact that the rate of transport of oxygen into the carbon sieve pore structure is much higher than that of nitrogen. Another factor that can affect the separation is the shape of gas molecules as compared to the shape of the opening in the adsorbent. The choice of molecular sieve as an adsorbent is therefore unobvious. The sieving material can be either a zeolite type or a carbon molecular sieve.
Methods for the regeneration of molecular sieves include pressure change, heating and purging with a carrier gas, or heating under vacuum conditions. Temperatures typically used to regenerate water-adsorbed molecular sieves typically range from 130° C. to 250° C.